In general, a MOS transistor, which is a type of field effect transistor, has a structure where source and drain regions are formed on a semiconductor substrate and a gate oxide film and a gate are formed on the semiconductor substrate on which the source and drain regions are formed.
Known example methods for forming a gate in a MOS transistor are disclosed in U.S. Pat. Nos. 6,548,388 and 6,063,699. In conventional methods for fabricating the MOS transistor, after a gate oxide film is formed in a device active region of a semiconductor substrate and a polycrystalline silicon layer is formed on the gate oxide film, a photosensitive film is applied, exposed and developed on the polycrystalline silicon layer to form a pattern of photosensitive film for covering the polycrystalline silicon layer only by a desired width and exposing the remaining polycrystalline silicon layer.
Next, using, for example, an EPD (end point detection) apparatus, the exposed polycrystalline silicon layer is etched using the pattern of photosensitive film as a mask, leaving the polycrystalline silicon layer of the desired width to be formed as a gate. However, because light is reflected from the polycrystalline silicon layer and so on below the photosensitive film when the photosensitive film is exposed and developed, it is difficult to precisely pattern the photosensitive film to a desired pattern dimension. This may cause a critical problem because a pattern dimension of the photosensitive film decreases as a device becomes more highly integrated.
To overcome the problem of fabricating a gate having a narrow width within a highly integrated device, an organic or inorganic anti-reflection coating is typically formed on the polycrystalline silicon layer.
However, even if such an anti-reflection coating is formed, there is a practical lower limit to a pattern dimension of the photosensitive film. With known photolithography processes, a gate of 0.18 μm width can be realized, but it is not currently possible to realize a gate of widths reduced to 0.15 μm, 0.13 μm, etc.